1. Technical Field
The present invention relates to an aqueous ink for ink jet recording.
2. Related Art
In the ink jet recording method, small droplets of ink are ejected from fine nozzles and attached to a recording medium to perform recording. This method has the characteristic of being able to record high resolution and high-quality images at a high speed with a relatively inexpensive apparatus.
In the ink jet recording method, there are an extremely large number of factors to be studied, including the properties of the ink used, the stability in recording, and the quality of images to be obtained, and studies are being carried out not only on the ink jet recording apparatuses but also on the inks to be used.
For example, JP-T-2006-520405 discloses an ink jet ink including latex particles to which a reactive surfactant is covalently bonded and which include a cross-linking agent, with the object of forming a good image on a recording medium by stably ejecting ink droplets.
In addition, JP-A-2003-128976 discloses an aqueous ink including a coloring material/resin composite particles obtained by dispersing a molecular color developer with a reactive dispersing agent and subsequently polymerizing a resin, with the object of improving the coloring property, glossiness, and fixability of the image to be formed and improving the ejection stability of the ink.
Furthermore, JP-A-2006-282989 discloses an aqueous dispersion for ink jet recording including polymer particles, which are emulsion-polymerized in the presence of a reactive surfactant, and a water-insoluble organic compound where a log P value (logarithm of the distribution coefficient of 1-octanol of the organic compound/water) is −1 to 11,with the object of improving printing density and marker resistance.
In the techniques described in JP-T-2006-520405, JP-A-2003-128976, and JP-A-2006-282989 described above, a reactive surfactant or a reactive dispersing agent is used to prepare latex particles, coloring material/resin composite particles, or polymer particles by emulsion polymerization. Such particles have a function of improving the fixability of recorded images. However, a defoaming agent is not added when these latex particles or coloring material/resin composite particles are used to form inks.
Here, in the emulsion polymerization method, since an emulsifier is essential at the time of polymerization and the emulsifier remains in the system after polymerization, the obtained aqueous dispersion has a property of foaming easily. Then, in a case where such an aqueous dispersion is used for ink for ink jet recording which is intermittently ejected (discharged) as liquid droplets, for example, cleaning operations or the like easily cause the ink to foam and it is also difficult to eliminate the generated bubbles. Therefore, minute bubbles are easily taken into the flow path of the recording head or the like, and the ejection property of the ink may deteriorate.
Reducing the amount of emulsifier during the polymerization of the resin particles may be considered as a method for making bubbles less likely to occur. However, when the added amount of the emulsifier is reduced during the emulsion polymerization as described above, the dispersibility of the resin particles is insufficient and it is difficult to obtain storage stability when used in an ink.
In addition, adopting a reactive surfactant able to function as an emulsifier at the time of reaction and incorporating the emulsifier into the particles after polymerization may be considered as a method for making bubbles less likely to occur without reducing the emulsifier. However, in order to reduce the remaining emulsifier, it is necessary to reduce the amount of unreacted (free) reactive surfactant and it is still difficult to obtain a reaction yield (conversion) of 100% in the polymerization of the reactive surfactant.
Under such circumstances, the use of a defoaming agent may be considered as a method for making bubbles less likely to occur and making the generated bubbles easily breakable without reducing the unreacted reactive surfactant and added emulsifier. There are many kinds of defoaming agents and the materials and defoaming mechanisms thereof are diverse. In many cases, defoaming agents have high hydrophobicity and combination with a defoaming agent may destroy the dispersed state of the resin particles dispersed in water and lead to the generation of aggregates. When such a defoaming agent is used, the aggregates become larger than a certain size and clog the recording head or ink flow path, which may cause ejection defects. In addition, depending on the kind thereof, the defoaming agent may be repelled from the printed surface and problems often occur if the defoaming agent is selected without considering the combination with other materials in the ink.
JP-A-2003-128976 describes the use of a defoaming agent, but does not suggest specific materials or combinations thereof and, in ink for ink jet recording in particular, there is a concern that the use of a water-insoluble organic compound having a log P value of approximately −1 to 11 as described in JP-A-2003-128976 as a defoaming agent may aggregate the resin particles and coloring material through combination with other materials in the ink, similarly to highly hydrophobic defoaming agents.
In this manner, even considering only the storage stability and ejection stability of the aqueous ink for ink jet recording, there are an extremely large number of factors to be examined. In addition, there is a need for well-balanced aqueous inks for ink jet recording which also improve the quality of obtained images.